d_{3z^2-r^2} Orbital in high-Tc cuprates: Excitonic spectrum, metal-insulator phase diagram, optical conductivity and orbital character of doped holes

TL;DR

This study uses dynamical mean-field theory to analyze the role of d_{3z^2-r^2} orbitals in high-Tc cuprates, revealing their limited impact on the Fermi surface and conductivity, and comparing T and T' crystal structures.

Contribution

It introduces a comprehensive model including both orbitals and oxygen states, providing new insights into their effects on phase stability and excitonic features in cuprates.

Findings

d_{3z^2-r^2} orbital broadens charge transfer insulator stability

Fermi surface remains single-sheet with negligible orbital admixture

Extra orbitals do not resolve conductivity discrepancies in insulators

Abstract

The single-site dynamical mean-field approximation is used to solve a model of high-Tc cuprate superconductors which includes both d_{x^2-y^2} and d_{3z^2-r^2} orbitals on the Cu as well as the relevant oxygen states. Both T (with apical oxygen) and T' (without apical oxygen) crystal structures are considered. In both phases, inclusion of the d_{3z^2-r^2} orbital is found to broaden the range of stability of the charge transfer insulating phase. For equal charge transfer energies and interaction strengths, the T' phase is found to be less strongly correlated than the T phase. For both structures, d-d excitons are found within the charge-transfer gap. However, for all physically relevant dopings the Fermi surface is found to have only one sheet and the admixture of d_{3z^2-r^2} into ground state wave function remains negligible (<5%). Inclusion of the extra orbitals is found not to resolve the discrepancy between computed and observed conductivity in the insulating state.